In a typical anti-lock braking system pump assembly, the pistons are diametrically opposed to one another and reciprocate along a single axis as driven by the pump drive motor. In this configuration, the pump generally produces a pressurized fluid only during one direction of the piston stroke. A relatively rotatable sealing contact type bearing is pressed onto the eccentric diameter of the drive motor shaft such that the operation of the drive motor causes the bearing to move in a circular path about the axis of the drive motor shaft. Since the pump pistons as driven by the motor through the bearing can move only along one axis, they will sense only the component of the bearing movement along that axis. For a constant angular speed of the motor shaft, the pistons will be displaced with a sinusoidal motion. The path which the two pumps follow can be graphed as shown in FIG. 1.
Since the pumps of the prior art only provide high pressure fluid as they are being stroked in one direction (away from the center of the motor shaft), the torque on the motor shaft can be depicted as shown in FIG. 2.
This can be better visualized by picturing the mechanical linkage of a piston and crank assembly, as shown in FIG. 3.
At the 0.degree. and 180.degree. positions (using the standard Cartesian coordinate system shown in FIG. 3, centered at the axis of the motor shaft), there is no torque transmission to the motor shaft in the diametrically opposed piston type pump of the prior art. The torque transmission will be a maximum at the angles of 90.degree. and 270.degree..
There are two primary reasons why an improvement over this design is sought. The first reason is that a cyclic torque distribution such as that shown in FIG. 2 increases the possibility of generating unwanted noise and vibration. The second reason relates to work output. The work done is proportional to the area under the curve shown in the torque graph. If a different drive mechanism were devised which flattened this curve, a constant torque level equal to one half the peak torque of the prior art curve would yield the same work done on a pump. The input energy required to drive the electric motor, however, will not be the same between the two cases. For a sinusoidal torque forcing function, as in the prior art, the average energy required to operate the motor is proportional to the rms (root means square) current draw on the motor, rather than the arithmetic average. The rms average for a pure sinusoidal curve is approximately 1.4 times the arithmetic average. Due to the inertial effects of the motor, however, a pure sinusoidal torque transmission will not be required by the motor. The actual power multiplication required to operate a motor driving the prior art design will be dependent upon the angular momentum of the motor's rotor.
A pump assembly utilizing four pumps is shown in FIG. 1.
Following the same logic as in the construction of the curves in the example shown in FIG. 2, the torque curve for the configuration of FIG. 4 would appear as shown in FIG. 5.
Due to the overlapping of these curves, the actual torque transmitted to the motor shaft will be a constant value: equal to the peak value shown in the graph (which is one half the peak value of the torque graph of the prior art design). This, therefore, optimizes the mechanical loading of the motor shaft and ensures that additional power losses will not be encountered in the electric motor. However, this design is impractical for two reasons. First, a much higher frictional load at the contact areas of the bearing race and the pistons is encountered in this design, compared to the design of the prior art, due to the fact that the bearing must now slide across the face of certain pistons while they are being loaded. Second, it is impractical for the size and cost of the unit to use a four-pump design.
It would be desirable to develop a drive mechanism using two pumps which attains the ideal loading condition (four power strokes per revolution occurring at 90.degree. intervals) with no penalties in cost, size or performance.
It would also be desirable to develop such a drive mechanism in which there is an improvement in cost, size and performance of the pump assembly.
It is also desirable to develop a dual-action hydraulic pump which produces pressurized fluid as the piston is stroked in either direction.
A development of a dual-action hydraulic pump would provide several advantages over the prior art. A dual-action hydraulic pump would provide smoother flow and pressure output, it would reverse the positions of the inlet and outlet ports in order to allow an integrated low pressure accumulator, and it would eliminate the piston return spring. Along with these improvements would come an improvement in pump efficiency which reduces power requirements and electronic component sizing.
Accordingly, an object of the present invention is to provide a pumping device for pumping fluid from an inlet to an outlet in which the piston pumps fluid from the pump as the piston moves in either direction.
A further object of the present invention is to provide a pump assembly with improved cost, size and performance characteristics.
Yet another object of the present invention is to provide a pump assembly for pumping fluid in an antilock braking system from a motor-driven orbitally rotating cam, wherein two pumps may be arranged in a side-by-side relationship, and the pistons of the two pumps may be translated by the single cam.
Other objects, uses and advantages of this invention are apparent from a reading of this description which proceeds with reference to the accompanying drawings.